In the radiation imaging system such as an X-ray imaging apparatus and a CT (Computerized Tomography) system, generally an X-ray source emits X-rays toward an inspection object or a subject such as a patient or goods. An X-ray beam is attenuated by the inspection object, and then is incident on radiation detectors arranged in an array form. A radiation intensity received in a position of each detector typically depends upon attenuation of X-rays. Each of the detection elements in detectors arranged in an array form generates an electric signal which represents an attenuated beam received by each detection element. These signals are transmitted to a data processing system for an analysis, and an image is finally created by the data processing system.
As for radiation detection, a scheme in which radiation is incident on a fluorescent material such as a scintillator and generated fluorescent light is detected by a photodiode, a photomultiplier tube, or the like is common. There is a property that the number of fluorescent photons emitted from a fluorescent material used at this time is proportionate to radiation energy incident upon the fluorescent material. Therefore, it becomes possible to measure energy of radiation which has been transmitted through an inspection object by counting the number of fluorescent photons emitted from the fluorescent material. It becomes possible to obtain a CT image based upon energy discrimination, i.e., a color CT image by applying this property to CT or the like.
For implementing a CT of photon detection type, detection of the number of fluorescent photons conducted by arranging avalanche photodiodes operating in a Geiger mode as detection elements and counting the number of photons incident on a detector is being studied. As for the avalanche photodiodes operating in the Geiger mode, utilization of a Si material and the like is being studied.
Each of the avalanche photodiodes (hereafter referred to as APDs as well) operating in the Geiger mode is a photodiode which emits one current pulse every photon incident on the photodiode. A current pulse having a wave height proportionate to the number of APDs on which photons are incident is emitted by arranging APDs in an array form. It becomes possible to measure the number of photons incident on the photon detector, i.e., energy of radiation incident on the fluorescent material by pulse height analysis.
As described later, it is found according to a study result of the present inventors that it is impossible to sufficiently take out photons in conversion from radiation to visible light and count visible light without omission, in a conventional radiation detection apparatus which counts X-ray photons by using a light sensing system obtained by combining a scintillator with an APD array.
In the conventional radiation detection apparatus, therefore, the detection efficiency of fluorescent photons emitted from a fluorescent material cannot be enhanced.